The present technology relates to a holographic data storage medium which can be used, for example, for storing image data such as photos, logos, text, and for the storage of other data.
In a hologram, optical phase information about an object is contained distributed over the surface of the hologram, from which an image of the object can be reconstructed when it is irradiated with light, in particular coherent light from a laser. Holograms are used in industry in many ways, for example in the form of largely counterfeit-proof identifications. Identifications of this type will be found, for example, on credit cards or cheque cards; as what are known as white light holograms, they show a three-dimensional image of the object represented even when lit with natural light. Photographically produced holograms and embossed holograms are widespread, in which a relief structure is embossed into the surface of a material, at which the light used to reproduce the object is scattered in accordance with the phase information stored in the hologram, so that the reconstructed image of the object is produced by interference effects.
WO 00/17864 describes a data storage medium having an optical information carrier which contains a polymer film set up as a storage layer. The polymer film consists, for example, of biaxially oriented polypropylene. In the previously disclosed data storage medium, the polymer film is wound spirally in a plurality of layers onto a core, there being an adhesive layer in each case between adjacent layers. Information can be written into the data storage medium by the polymer film being heated locally with the aid of a write beam from a data drive, as a result of which the refractive index of the polymer film and the reflective capacity at the interface of the polymer film change locally. This can be registered with the aid of a read beam in the data drive, since the read beam is reflected locally more or less intently in the interface of the polymer film, depending on the information written in. By focusing the write beam or read beam, information can specifically be written into and read from a preselected layer of the information carrier.
The present technology provides a holographic data storage medium which is cost-effective and has wide possible applications.
This object is achieved by a holographic data storage medium having the features of Claim 1 and the use of a data storage medium according to Claim 11. A method of putting information into such a data storage medium is specified in Claim 13, a method of reading information from such a data storage medium in Claim 17. Advantageous refinements of the invention are listed in the dependent claims.
The holographic data storage medium according to the an exemplary embodiment of the invention has a polymer film set up as a storage layer, whose refractive index can be changed locally by heating. The polymer film is set up to store optical phase information by means of the local optical path length in the polymer film, which is illuminated in transmission when reading out information. In the polymer film, it is therefore possible to store phase information locally, that is to say in a region provided for the storage of a unit of information, by the refractive index being changed by heating in this region. The local change in the refractive index effects a change in the optical path length of the radiation used when reading information out of the polymer film (the radiation illuminating the polymer film in transmission). This is because the optical path length is the product of the geometric path length and the refractive index; by means of a change in the refractive index, therefore, the local phase angle of the radiation used when reading out information may be influenced, that is to say the desired phase information may be stored. The local region for storing a unit of information (referred to as a “pit” in the following text) typically has linear dimensions (that is to say, for example, a side length or a diameter) of the order of magnitude of 0.5 μm to 1 μm, but other sizes are also possible.
The holographic data storage medium according to an exemplary embodiment of the invention may therefore be used as a refractive phase hologram or transmission hologram. In this case, the desired phase information is stored locally via a change in the refractive index and, in order to read out information, the polymer film is transilluminated. As opposed to this, in the data storage medium disclosed by WO 00/17864, the reflective capacity at the interface of the polymer film is changed, in order to be able to register differences in the amplitude of a reflected read beam during the read operation. The holographic data storage medium according to an exemplary embodiment of the invention is cost-effective and can be applied in many ways, as emerges from the following text.
In the holographic data storage medium according to an exemplary embodiment of the invention, the polymer film is preferably biaxially oriented, for example by being prestressed in two mutually perpendicular directions within its plane during production. In an oriented polymer film, a high energy density is stored in the film material. As a result of heating by depositing a relatively low quantity of energy per unit area, for example with the aid of a write beam, a relatively high material change (for example densification of material) by reformation can be achieved, which results in a local change in the refractive index and therefore in a change in the optical path length in the polymer film. Biaxially oriented polymer films may be produced from mass-produced plastics, so that the holographic data storage medium is cost-effective.
Suitable materials for the polymer film are, for example, polypropylene, polyester or polyvinylchloride, polymer films which have such a material preferably being biaxially oriented. High temperature stability and therefore an improved resistance to ageing and storage stability of the holographic data storage medium, and an increased security against data loss arising from ageing processes may be achieved with polymer films which have an elevated crystallite melting point. In this case, the crystallite melting point is preferably at least 170° C. Examples of such materials are polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polymethylpentene (PMP; also poly-2-methylpentene) and polyimide, a polymer film from such materials preferably also being biaxially oriented.
Preferred thicknesses of the polymer film lie in the range from 10 μm to 100 μm, preferably around or below 50 μm, but also smaller and greater thicknesses are conceivable.
In a preferred refinement of the invention, a reflective layer (for example of aluminium) is arranged behind the polymer film. The radiation used to read out information, that is to say in particular laser light, is thrown back at this reflective layer, so that the polymer film is illuminated twice in transmission when information is read out. The distance between the pits and the reflective layer (that is to say the thickness of the zone of the polymer film which adjoins the reflective layer and in which the refractive index is not changed locally) is preferably set up such that disruptive interference and superimposition effects are avoided. The configuration of the holographic data storage medium with a reflective layer behind the polymer film has the advantage that the arrangement can be fitted to a mechanical carrier or else directly to an object provided with information, since the image representing the stored information does not have to be reconstructed behind the polymer film.
To the polymer film, there can be assigned an absorber dye, which is set up to at least partly absorb a write beam serving to input information and to give up the heat produced in the process at least partly locally to the polymer film. An absorber dye of this type permits local heating of the polymer film which is sufficient to change the refractive index at a relatively low intensity of the write beam. The absorber dye is preferably arranged in an absorber layer arranged on the polymer film. It can also be admixed to the material of the polymer film; mixed forms are likewise conceivable. The absorber layer preferably has a thin layer (for example a thickness of 1 μm to 5 μm) of an optically transparent polymer (for example of polymethyl methacrylate (PMMA) or, in the case of applications for higher temperature, of polymethylpentene, polyetheretherketone (PEEK) or polyetherimide), which serves as a matrix or binder for the molecules of the absorber dye. The absorption maximum of the absorber dye should coincide with the optical wavelength of the write beam used, in order to achieve efficient absorption. For an optical wavelength of 532 nm of a write beam produced by a laser, for example dyes from the Sudan red family (diazo dyes) or eosin scarlet are suitable. For the common laser diodes with an optical wavelength of 665 nm or 680 nm, green dyes, for example from the styryl family (which are common as laser dyes), are more suitable.
In a preferred refinement of the invention, the holographic data storage medium has an adhesive layer for sticking the data storage medium to an object. The adhesive layer makes it possible to stick the data storage medium quickly and without difficulty to a desired object, for example to use the data storage medium as a machine-readable label in which information about the object is stored. Particularly suitable as an adhesive layer is a self-adhesive layer or a layer with a pressure-sensitive adhesive, which, in the delivered state of the data storage medium, is preferably provided with a protective covering that can be pulled off (for example of a film or a silicone paper).
Apart from the previously mentioned layers, the data storage medium can also have additional layers, for example a protective layer of a transparent varnish or polymer which is arranged in front of the polymer film or the absorber layer, or a mechanical carrier which is located behind the reflective layer. An optional adhesive layer is preferably arranged behind the reflective layer or behind the mechanical carrier.
Information to be stored can be input into the holographic data storage medium by means of a method in which phase information contained in a hologram of a storing object is calculated as a two-dimensional arrangement and a write beam from a writing device, preferably a laser lithograph, is aimed at a storage layer and/or possibly the associated absorber layer of the data storage medium and is driven in accordance with the two-dimensional arrangement in such a way that the local optical path length in the polymer film set up as a storage layer is set by a local change in the refractive index in accordance with the phase information. Since the physical processes in the scattering of light at a storing object are known, a conventional set-up for producing a hologram (in which, for example, coherent light from a laser, which is scattered by an object (storing object) is brought into interference with a coherent reference beam and the interference pattern produced in the process is recorded as a hologram) is simulated with the aid of a computer program, and the interference pattern and the phase information contained therein are calculated as a two-dimensional arrangement (two-dimensional array). The resolution of a suitable laser lithogragh is typically about 50 000 dpi (dots per inch). The refractive index in the polymer film can therefore be changed locally in regions or pits of a size of about 0.5 μm to 1 μm. The write speed and other details depend, inter alia, on the parameters of the write laser (laser power, optical wavelength) and the exposure duration.
The phase information is therefore preferably input into the storage layer in the form of pits of predefined size. In this case, the phase information can be stored in a pit in binary encoded form. This means that, in the region of a given pit, the polymer film assumes only one of two possible values for the refractive index. These values preferably differ considerably, in order that intermediate values occurring in practice for the refractive index which lie close to one or the other value can be assigned unambiguously to one or the other value, in order to store the information reliably and unambiguously.
Alternatively, the phase information can be stored in continuously encoded form in a pit, the local optical path length in the pit being selected from a predefined value range. This means that, in a given pit, the refractive index of the polymer film can assume any desired value from a predefined value range. In this case, the information may therefore be stored “in grey stages”, so that each pit is given the information content from more than one bit.
In a method of reading information out of a holographic data storage medium, light, preferably coherent light (for example from a laser) is aimed over a large area onto a storage layer of the data storage medium, and the storage layer of the data storage medium is illuminated in transmission, the light possibly being reflected at the reflective layer (if one such is present) behind the polymer film set up as a storage layer. As a reconstruction of the information contained in the illuminated region, a holographic image is registered at a distance from the data storage medium, for example by using a CCD sensor which is connected to a data processing device.
The term “large area” is to be understood to mean an area which is considerably larger than the area of a pit. In this sense, for example, an area of 1 mm2 is a large area. For the scheme according to which information is stored in a holographic data storage medium according to the invention and read out, there are many different possibilities. It is conceivable to read out from the data storage medium in one operation, by the entire area of the polymer film set up as a storage layer being illuminated in one operation. In particular in the case of larger areas, however, it is advantageous to divide up the information to be stored into a number or large number of individual regions (for example with a respective area of 1 mm2) and to read out the information only from a predefined individual area in one operation.
When information is read out, the illuminated region of the polymer film acts as a diffraction grating, the incident light being deflected in a defined manner as a result of the locally varying refractive index or optical path length. The deflected light forms a holographic image of the stored object. This image represents the reconstruction of the information encoded via the varying local optical path length (refractive index modulation).
The holographic data storage medium can be used for different types of stored objects. For example, both the information contained in images, such as photographs, logos, texts, and so on, and machine-readable data can be stored and read out. The latter is carried out, for example, in the form of data pages, as they are known, the phase information contained in a hologram of a graphic bit pattern (which represents the data information) being input into the polymer film as explained. When the said data is read out, a holographic image of this graphic bit pattern is produced. The information contained therein can be registered, for example with the aid of an accurately adjusted CCD sensor, and processed by associated evaluation software. For the reproduction of images, in which high accuracy is not an issue, in principle even a simple matt disc, or, for example, a camera with an LCD screen is sufficient.
In the case of the holographic storage of machine-readable data, it is advantageous that the information does not have to be read out sequentially but that an entire data set can be registered in one operation, as explained. Should the surface of the storage layer be damaged, then, as opposed to a conventional data storage medium, this does not lead to a loss of data but only to a worsening of the resolution of the holographic image reconstructed when the information is read out, which is generally not a problem.